This idea of the "truth of the third" is how some people may refer to
the idea of unlearned dissonance has been around for a long time,
coming out of the last three centuries of western european tonal
music.

The third is imperfect, and the fourth is perfect, circa 1100.
Somehow the fourth became dissonant. Listening to the musics of both
Bulgaria and the Georgian Republic has taught me much about
non-western european concepts of dissonance / consonance, and feeds
my view that a number of psychoacoustic ideas are learned, and can be
unlearned.

But I'm not a strong believer in a 'divine path' of "false
beginnings". Webern and Schoenberg (et al) forever changed consonance
/ dissonance. A younger generation has grown up with other values
regarding where this boundary lies, in my experience. I may hear
beating, and roughness at 15Hz beating, but this is not dissonant to
me, for I hear dissonance as a psychometric measurement -- it is not
inherent in the acoustical signal.

My reading of the original question was that a non-cultural,
non-learned metric was being sought for a cultural, learned metric.

to learn something is not the biological purpose of a human brain.
Its purpose is to keep up a workable equilibrium. If acoustical
signals, such as music by Webern, can be of use for a brain depends
on numerous things.

Neither in music nor in psychoacoustics can there be something like
absolute consonance or absolute dissonance. It's always relative to
something else.

The idea that consonance can be learned, and that dissonance can be
unlearned, has been a popular one in some circles for quite some
time. In my view this idea is just one of the many "false
beginnings" that have cropped up during the course of human culture.